Degradation of Ga2O3–In2O3–ZnO (GIZO) thin-film transistors (TFTs), which are promising for driving circuits of next-generation displays, was studied. We found a degradation mode that was not observed in silicon TFTs. A parallel shift without any change of the transfer curve was observed under gate voltage stress. Judging from the bias voltage dependences we confirmed that the mode was mainly dominated by a vertical electric field. Thermal distribution was measured to analysis the degradation mechanism. Joule heating caused by drain current was observed; however, a marked acceleration of degradation by drain bias was not found. Therefore, we concluded that Joule heating did not accelerate degradation. Recovery of electrical properties independent of stress voltage were observed.

Degradation of Ga 2 O 3 -In 2 O 3 -ZnO (GIZO) thin-film transistors (TFTs), which are promising for driving circuits of next-generation displays, was studied. We evaluated degradation caused by applying gate voltage and drain voltage stress. A parallel shift of the transfer curve was observed under gate voltage stress. The amount of threshold voltage shift when applying gate and drain voltage stress was smaller than that in the case of only gate voltage stress. Joule heating caused by the drain current was observed. We reproduced this degradation of transfer curve change by device simulation. When we assumed the trap level as the density of state (DOS) model and increased two kinds of trap density, we obtained properties that show the same trends as the experimental results. We concluded that two degradation mechanisms occur under gate and drain voltage stress conditions.

Amorphous In-Ga-Zn-O (a-IGZO) thin-film transistors on glass substrates were fabricated, and the density of state (DOS) in TFTs after a negative bias stress (NBS) and negative bias illumination stress (NBIS) was investigated. For a NBIS of −20 V, the threshold voltage (V th ) shifted toward the negative direction but an increase of shallow states expected as positively ionized oxygen vacancies (V o + or V o ++ ) were not observed in the DOS curves. On the other hand, for NBIS of −30 V, changes in the DOS near the conduction band edge before and after applying a bias stress were observed. This suggests that V o + or V o ++ was generated by NBIS but accumulation of holes at the a-IGZO/SiO 2 interface mainly caused the V th shifts.Amorphous indium gallium zinc oxide (a-IGZO) is a transparent amorphous oxide semiconductor and is expected to be used as a channel material in thin film transistors (TFTs) for next-generation displays such as active matrix organic light-emitting diode displays. 1 Due to the electronic structure, a-IGZO TFTs have high electron fieldeffect mobility in spite of the material being amorphous. Moreover, it is easy to control the increase of the superfluous carrier density in a-IGZO TFTs compared to the case for TFTs fabricated from ZnO, which is a similar oxide semiconductor. 2 Moreover, very low off-state current is realizable because holes hardly exist. 3 Although a-Si is deposited by plasma enhanced chemical vapor deposition (PECVD) at approximately 350 • C, a-IGZO can be deposited by sputtering process at room temperature. Since a-IGZO TFTs are promising devices for low-temperature process, the application of a-IGZO TFTs to flexible electronics using a plastic substrate is expected. 4 In addition, TFTs with transparency in the visible wavelength are producible because of the wide bandgap. 5,6 The electrical stability of a-IGZO-TFTs under a positive bias stress (PBS) and a negative bias stress (NBS) has been reported by various researchers for practical applications. 7-14 In recent years, the negative shift of the threshold voltage (V th ) under negative bias illumination stress (NBIS) has been under focus. [15][16][17][18] Since TFTs are continually exposed to light irradiation in displays, for example backlight irradiation in liquid crystal displays, their reliability under illumination is a critical problem. Recently, some groups have reported that positively ionized oxygen vacancies (V olayers increased under NBIS, and these vacancies caused a negative shift in V th . 19 V o ++ and V o + are known as conventional defects in ZnO, a material similar to a-IGZO. 20,21 It has been proposed that V o ++ and V o + in a-IGZO exist shallow levels below the conduction band minimum and are derived from high concentration of natural vacancies (V o , 10 21 cm −3 , Ref. 22) that produce defect states near the valence band maximum.On the other hand, other groups have suggested diffusion of photoexcited holes to the SiO 2 /a-IGZO interface as the reason for the negative V th shift. 23 In addition, anothe...

In this study, we successfully achieved a relatively high field-effect mobility of 37.7 cm 2 /Vs in an InZnO thin-film transistor (TFT) fabricated by excimer layer annealing (ELA). The ELA process allowed us to fabricate such a high-performance InZnO TFT at the substrate temperature less than 50 C according to thermal calculation. Our analysis revealed that high-energy irradiation in ELA produced a mixed phase of InZnO and SiO 2 , leading to the deterioration of TFT characteristics. Oxide semiconductor films deposited by sputtering have recently attracted considerable attention in the fields of transparent and flexible electronics for next-generation displays, in comparison with conventional amorphous silicon-based materials. In particular, an In 2 O 3 -doped ZnO (IZO) thin film is widely recognized as a suitable oxide semiconductor since thin-films transistors (TFTs) with that material in the channel layer yield a higher field-effect mobility than amorphous InGaZnO (a-IGZO) TFTs. [1][2][3] In the case of the IZO film, the amorphous phase generally provides a high conductivity of about 400 X À1 cm À1 , which is much higher than the conductivity suitable for the TFT's channel layer. 4,5 The conductivity of this material can possibly be reduced by a thermal annealing process. 6,7 In previous reports, thermal annealing with a relatively high temperature of about 300 C has been proposed to produce operative IZO TFTs. 8,9 However, such a high-temperature process in postannealing limits the choice for plastic flexible substrates. The excimer laser annealing (ELA) process with short pulses has been widely utilized to achieve a low processing temperature to produce a polycrystalline silicon thin film on a glass substrate. 10 This technique has also been utilized for oxide-semiconductor materials. Nakata et al. reported the fabrication of a-IGZO TFTs by ELA process, which acted as a postannealing process, and the improvement of the TFTs characteristics by ELA, 11 suggesting that the ELA process is also a promising technique for improving the characteristics of oxide-semiconductor devices. In this study, we focused on an IZO TFT with a higher field-effect mobility than an a-IGZO TFT. We assumed that an ELA process crystallizes IZO with nanograins easily since this material has a lower crystallization temperature than a-IGZO 12,13 and that an IZO film composed of nanograins has better characteristics than a noncrystallized IZO TFT since carrier scattering is suppressed in the disordered film. We considered it is necessary to use a laser with a wavelength shorter than 400 nm, for the laser to be absorbed by the IZO film since IZO has a wide band gap (higher than 3 eV). 14 We used an XeCl excimer laser with a sufficiently short wavelength of 308 nm (photon energy: 4.02 eV) 15,16 and investigated the effects of the ELA process on the IZO TFT characteristics and film properties.A thermally oxidized SiO 2 film of 100 nm thickness was formed on a highly doped p-type silicon wafer (<0.002 X cm) as a gate insulator. IZO films with...

Transparent amorphous oxide semiconducting materials such as amorphous InGaZnO used in thin film transistors (TFTs) are typically annealed at temperatures higher than 250 °C to remove any defects present and improve the electrical characteristics of the device. Previous research has shown that low cost and low temperature methods improve the electrical characteristics of the TFT. With the aid of surface and bulk characterization techniques in comparison to the device characteristics, this work aims to elucidate further on the improvement mechanisms of wet and dry annealing ambients that affect the electrical characteristics of the device. Secondary Ion Mass Spectrometry results show that despite outward diffusion of –H and –OH species, humid annealing ambients counteract outward diffusion of these species, leading to defect sites which can be passivated by the wet ambient. X-ray Photoelectron Spectroscopy results show that for devices annealed for only 30 min in a wet annealing environment, the concentration of metal-oxide bonds increased by as much as 21.8% and defects such as oxygen vacancies were reduced by as much as 18.2% compared to an unannealed device. Our work shows that due to the oxidizing power of water vapor, defects are reduced, and overall electrical characteristics are improved as evidenced with the 150 °C wet O2, 30 min annealed sample which exhibited the highest mobility of 5.00 cm2/V s, compared to 2.36 cm2/V s for a sample that was annealed at 150 °C in a dry ambient atmospheric environment for 2 h.

Lysophosphatidic acid (LPA), which is a bioactive phospholipid, interacts with specific G protein-coupled transmembrane receptors. Recently, alterations of LPA receptor genes have been reported in some tumor cells. In this study, we examined the expression profiles and DNA methylation status of LPA receptor 1-5 (LPA1-5) genes in human colon cancer cells and also looked for the mutations. Reverse transcription-polymerase chain reaction (PCR) and bisulfite sequencing analyses were carried out. While LPA1, LPA2, and LPA4 genes were expressed in DLD1, SW480, HCT116, CaCo-2, SW48, and LoVo cells, the expressions of LPA3 and LPA5 genes were various. These expression levels were correlated with DNA methylation status in the 5' upstream regions of the LPA receptor genes. Mutation analysis was also performed using a PCR-single-strand conformation polymorphism method. Although no mutations in LPA1, LPA3 and LPA5 genes were found in all types of cells, LPA2 mutations in DLD1 and SW48 cells, and LPA4 mutation were found in DLD1 cells. On the basis of the present results, we demonstrate that these colon cancer cells will be available to understanding the molecular pathway through LPA receptors in the development of tumor cells, and that LPA receptors may be new molecular targets for therapeutic approaches and chemoprevention.

For gate insulator of amorphous InGaZnO thin-film transistor, we fabricated fluorinated silicon nitride (SiN:F) film formed by an inductively-coupled plasma enhanced chemical vapor deposition method by utilizing SiF4/N2 as source gases. Threshold voltage shift against electrical stress was successfully suppressed. Chemical analysis revealed that the hydrogen concentration was reduced to 1/10 of conventional SiN film and fluorine was introduced into the interface between the SiN:F film and channel layer. We conclude that the decrease of hydrogen content and introduction of fluorine lead to decrease of electron trap density at the interface and/or the SiN:F film.

We report the fabrication of a photosensitive hybrid passivation material on amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) that greatly enhance its stability and improve its electrical characteristics. The hybrid passivation based on polysilsesquioxane is transparent and fabricated using a simple solution process. Because the passivation is photosensitive, dry etching was never performed during TFT fabrication. TFTs passivated with this material had a small threshold voltage shift of 0.5 V during positive bias stress, 0.5 V during negative bias stress, and −2.5 V during negative bias illumination stress. Furthermore, TFTs passivated by this layer were stable after being subjected to high relative humidity stress — confirming the superb barrier ability of the passivation. Analysis of secondary ion mass spectrometry showed that a large amount of hydrogen, carbon, and fluorine can be found in the channel region. We show that both hydrogen and fluorine reduced oxygen vacancies and that fluorine stabilized weak oxygen and hydroxide bonds. These results demonstrate the large potential of photosensitive hybrid passivation layers as effective passivation materials.